Added diff_rot function.
TESTS!
Tidy
Change to timedelta object
Howard Reference
more Reference
Finished modifications and added Snodgrass paramters
Author modification
Merge branch 'diff_rot' of https://github.com/Cadair/sunpy into diff_rot

Conflicts:
	sunpy/coords/util.py
Merge pull request #401 from Cadair/diff_rot

Tided version of previously submitted diff_rot code